An image printing apparatus such as a printing apparatus, printer, or copying machine using a plurality of ink plates, e.g., Y (Yellow), M (Magenta), C (Cyan), and K (black) ink plates often prints respective images with different textures in order to prevent interference between the ink plates. The texture means lines, dither, and dots generally used in the image printing apparatus.
As a method of changing the texture, e.g., the angle of the texture is changed to Y=0°, M=15°, C=75°, and K=45° for the respective ink plates without changing the period. As another method, not only the angle but also the frequency is changed.
However, changing the frequency or angle of the texture generates a difference in density stability, smoothness, or granular noise depending on the characteristics of an image processing apparatus.
For example, for lines, a line having an angle of 90° i.e., vertical line is resistant to jitter noise (generated by paper feed nonuniformity or the like) having a noise component perpendicular to the vertical line. To the contrary, an almost horizontal line is susceptible to this jitter noise.
If a texture poorest in characteristics (most susceptible to noise) is assigned to the most dominant tincture in an input original (e.g., M (Magenta) dominant over the flesh color in a figure image), the entire image quality degrades.
In many cases, an original input to the image processing apparatus has a texture when the input original is created by the image processing apparatus. In this case, the texture of the input original and that of an image output from the image processing apparatus interfere with each other to generate moiré fringes.